Synthesis And Properties Of Photovoltaic Materials Based On Benzotrithiophene Derivatives

Clean energy has to be explored for energy crisis. As we all know, solar energy isconsidered as a reliable way to solve energy demand in future. Therefore, organicsolar cells(OSCs) have been intensively studied in recent years, but the photoelectricconversion efficiency(PCE) of OSCs still has not met the standard for commercialapplications, so it should be deeply studied in future. In the past years, polymer solarcells(PSCs) based on benzo[1,2-b:3,4-b’:6,5-b’’]trithiophene (BTT) aselectron-donating units have shown good photovoltaic performance. In this thesis, anew BTT derivitive (benzo[1,2-b:3,4-b’:5,6-b’’]trithiophene) has been synthesied aselectron-donating units for organic photovoltaic materials, and their structure andproperties have been investigated carefully.1、 Intermediate compound2,4,6-trichlorobenzene-1,3,5-tricarbaldehyde wassynthesized from1,3,5-trichlorobenzene by Friedel-Craft reaction and then reductionreaction, then it reacted with ethyl2-mercaptoacetate to form the derivitive ofbenzo[1,2-b:3,4-b’:5,6-b’’]trithiophene. The isomer of benzo[1,2-b:3,4-b’:5,6-b’’]-trithiophene was prepared by the hydrolysis and heat decarboxylation reactions.Finally, the monomer of5,8-di(trimethyltin)-2-octyl-benzo[1,2-b:3,4-b’:5,6-b’’]-trithiophene (M1) was obtained. For comparison, another similar monomer (M2)based on benzo[1,2-b:3,4-b’:6,5-b’’]trithiophene was synthesised from2,3-dibromothiophene. Then, M1and M2were copolymered with4,7-bis-(2-bromo-3-hexylthiophen-5-yl)benzo[1,2,5]thiadiazole (DTBT) to obtain theconjugated polymers P1and P2, respectively. P1shows better solubility and highermolecular weight, which is beneficial to good solution processability. Although P1shows a blue and narrow absorption spectrum, the PSC based on P1possesses ahigher Jscvalue for its high molecular weight. Simultaneously, the P1-based PSCprovide a higher Vocdue to a deeper HOMO energy level of P1. Therefore, it exhibitsa PCE of0.78%, which is higher than that of P2(0.58%).2、Two A-D-A small molecules D1and D2were desigined and synthesized byintroducing benzo[1,2-b:3,4-b’:5,6-b’’]trithiophene as electron-donating unit,3-ethylrhodanine as elecrton-withdrawing units, and two thiophenes or three thiophenes asconjugated π-bridge, respectively. Compared with D2, D1shows a higher molarabsorption coefficient and a red shift absorption spectrum. In solid film state, D1shows obviously J-aggregation and a higher fluorescence intensity， but D2film onlyshows a one-seventh fluorescence intensity, which indicates there is strong fluorescent quenching in D2film. Additionally, the film of the complex of two small moleculesand PC61BM shows a nearly quenched emisson, which indicates that an effictivecharge transfer between small molecules and PC61BM can be achieved. The BHJdevice based D1showed a open-circuit voltage0.99V and a low PCE of0.83%whenchlorobenzene was used as solvent and the weight ratio of D1/PCBM was1:2, but thePCE value was improved to2.1%, accompanied with a high Vocvalue of1.1V and ahigh FF of0.45, when chloroform was used as solvent and the weight ratio ofD1/PC61BM was1:0.5.